CVE-2026-4167: Belkin F9K1122 Buffer Overflow Vulnerability

CVE-2026-4167 Overview

A stack-based buffer overflow vulnerability has been identified in the Belkin F9K1122 wireless range extender running firmware version 1.00.33. The vulnerability exists within the formReboot function located in the /goform/formReboot endpoint. An attacker can exploit this flaw by manipulating the webpage argument, causing a stack-based buffer overflow condition. This vulnerability can be exploited remotely over the network, potentially allowing an attacker to execute arbitrary code or crash the device. The exploit has been publicly disclosed, increasing the risk of active exploitation in the wild. The vendor was contacted regarding this vulnerability but did not respond.

Critical Impact

Remote attackers can exploit this stack-based buffer overflow to potentially gain complete control over the Belkin F9K1122 device, leading to network compromise, denial of service, or use of the device as a pivot point for further attacks.

Affected Products

• Belkin F9K1122 Wireless Range Extender
• Firmware Version 1.00.33

Discovery Timeline

• 2026-03-16 - CVE-2026-4167 published to NVD
• 2026-03-16 - Last updated in NVD database

Technical Details for CVE-2026-4167

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The formReboot function in the Belkin F9K1122 firmware fails to properly validate the length of user-supplied input passed through the webpage argument. When an attacker provides an overly long string, the function writes beyond the allocated stack buffer, overwriting adjacent memory including return addresses and other critical stack data.

The network-accessible nature of this vulnerability means that any attacker who can reach the device's web management interface can attempt exploitation. The attack requires low privileges (authenticated access to the web interface) but no user interaction is necessary, making it particularly dangerous in environments where these devices are accessible on local networks or, worse, exposed to the internet.

Root Cause

The root cause of this vulnerability is a failure to implement proper bounds checking on the webpage parameter before copying it into a fixed-size stack buffer. The firmware does not validate the length of incoming data against the destination buffer's capacity, allowing attackers to overflow the buffer and corrupt adjacent stack memory.

This is a classic embedded device vulnerability pattern where resource-constrained firmware developers may omit input validation to save code space or processing time, inadvertently introducing severe security risks.

Attack Vector

The attack is network-based and targets the web management interface of the Belkin F9K1122. An authenticated attacker sends a specially crafted HTTP request to the /goform/formReboot endpoint with an oversized webpage parameter value. The malicious payload overflows the stack buffer, potentially allowing:

1. Denial of service by crashing the device
2. Code execution by overwriting the return address with attacker-controlled data
3. Full device compromise if exploitation achieves remote code execution

The vulnerability mechanism involves sending a crafted POST request to the /goform/formReboot endpoint with an excessively long webpage parameter. This overflows the stack buffer in the formReboot function, corrupting adjacent memory structures. Technical details and proof-of-concept information can be found in the GitHub Belkin PoC Repository.

Detection Methods for CVE-2026-4167

Indicators of Compromise

• Unexpected device reboots or crashes of Belkin F9K1122 range extenders
• Anomalous HTTP POST requests to /goform/formReboot containing unusually long webpage parameter values
• Network traffic showing large payloads directed at the device's web management interface
• Device behavior changes indicating potential compromise or backdoor installation

Detection Strategies

• Implement network intrusion detection rules to identify HTTP requests to /goform/formReboot with oversized parameters
• Monitor for buffer overflow signatures in network traffic targeting embedded device management interfaces
• Deploy web application firewall rules to block requests with abnormally long form parameters to IoT devices
• Analyze device logs for repeated failed requests or crash events associated with the reboot functionality

Monitoring Recommendations

• Enable logging on network perimeter devices to capture all traffic to/from Belkin F9K1122 devices
• Implement regular device health checks to detect unexpected reboots or configuration changes
• Use network segmentation to isolate IoT devices and monitor inter-segment traffic for anomalies
• Deploy endpoint detection on network management systems that interact with these devices

How to Mitigate CVE-2026-4167

Immediate Actions Required

• Restrict network access to the Belkin F9K1122 web management interface using firewall rules or network segmentation
• Disable remote management access if not required for operations
• Place affected devices behind a VPN or access control system to limit exposure
• Monitor for exploitation attempts using the detection methods described above

Patch Information

At the time of disclosure, Belkin has not responded to vulnerability notifications and no official patch is available. Users should monitor VulDB #351074 and the vendor's support channels for potential firmware updates. Consider replacing affected devices with actively maintained alternatives if the vendor remains unresponsive.

Workarounds

• Implement network access controls to restrict access to the device's web interface to trusted IP addresses only
• Disable the web management interface if device functionality permits
• Deploy a reverse proxy with input validation in front of the device to filter malicious requests
• Isolate the device on a separate VLAN with strict ingress and egress filtering

# Example iptables rules to restrict access to Belkin device management interface
# Replace 192.168.1.100 with your Belkin F9K1122 IP address
# Replace 192.168.1.50 with trusted management workstation IP

# Block all incoming HTTP/HTTPS to the device
iptables -A FORWARD -d 192.168.1.100 -p tcp --dport 80 -j DROP
iptables -A FORWARD -d 192.168.1.100 -p tcp --dport 443 -j DROP

# Allow only trusted management IP
iptables -I FORWARD -s 192.168.1.50 -d 192.168.1.100 -p tcp --dport 80 -j ACCEPT